vce physics unit 3 topic 2
Post on 04-Jan-2016
Embed Size (px)
DESCRIPTIONVCE PHYSICS Unit 3 Topic 2. ELECTRIC POWER. Unit Outline. This unit covers the following areas: Apply a field model to magnetic phenomena including shapes and directions produced by bar magnets and by currents in wires, coils and solenoids - PowerPoint PPT Presentation
Unit OutlineThis unit covers the following areas:Apply a field model to magnetic phenomena including shapes and directions produced by bar magnets and by currents in wires, coils and solenoidsCalculate magnitudes, including determining the directions of, and magnetic forces on current carrying wires using F = nIlB, where the direction of I and B are either perpendicular to, or parallel to, each other.Investigate and explain the operation of simple DC motors consisting of one coil, containing a number of loops of wire, which is free to rotate about an axis - 2 magnets (not including radial magnets) - a commutator- a DC power supply4. Apply a field model to define magnetic flux using = BA and the qualitative effect of differing angles between the area and the field.5. Investigate and analyse the generation of emf, including AC voltage and calculations using induced emf = -N d/dt in terms of - rate of change of magnetic flux (Faradays Law) - the direction of the induced current (Lenzs Law) - the number of loops through which the flux passes. 6. Explain the production of DC voltages in DC generators and AC voltages in alternators including in the use of commutators and slip rings respectively7. Compare DC motors, DC generators and AC alternators 8.Investigate and compare sinusoidal AC voltages produced as a result of uniform rotation of a loop in a constant magnetic flux in terms of frequency, period, amplitude, peak to peak voltage and peak to peak current.9.Identify RMS voltage as an AC voltage which produces the same power in a resistive component as a DC voltage of the same magnitude.10.Convert between rms, peak and peak to peak values of voltage and current.11.Analyse transformer action, modelled in terms of electromagnetic induction for an ideal transformer. N1/N2 = V1/V2 = I1/I212.Analyse the supply of power as P = VI and transmission losses using potential transmission lines (V = IR) and power loss (P = I2R)13. Explain the use of transformers in an electricity distribution system.14. Identify and apply safe and responsible practices when working with electricity and electrical measurement
Chapter 1Topics covered:Magnetic FieldsFields around permanent magnetsFields around current carrying wiresFields around Solenoids.North Pole of a Solenoid.
1.0 Magnetic FieldsPROPERTIES OF MAGNETIC FIELDS1. Field Lines form Closed LoopsPermanent MagnetField Lines2. Field Lines NEVER Cross3. Spacing between Field Linesindicates Field Strength4. Direction of Field = Direction of Force on the Unit North Pole All magnets have poles labelled as North and South:Like poles repel Unlike poles attractMagnets generate Fields in the space surrounding them.The concept of a field is an important concept in our study of Physics.A Field is defined as a region of influence.In a Magnetic Field, magnetically susceptible materials are subject to an influence. They will experience a force when placed in the field.The strength of the magnetic field is determined by the size of the force experienced by a Unit North Pole* placed at the point of interest.*Does not yet exist, but physicists hope to produce one soon.
1.1 Magnetic Fields around Permanent Magnets Used in diagrams to show
current or field directions
Electric Power RevisionQuestion Type:The left side of Figure 6 shows three sources of magnetic fields.The right side of Figure 6 shows three possible magnetic field patterns of the shaded planes.
Q1: For each of the three sources, draw a line linking the source to the magnetic field pattern it produces in theshaded region.
1.2 Magnetic Field around A Current Carrying WireAny wire carrying an Electric Current has a Magnetic Field around itThe direction of the Magnetic Field can be determined using the: Right Hand Grip Rule
Electric Power RevisionQuestion Type:Q2: Draw the lines representing the magnetic field resulting from the straight current-carrying conductor in the figure opposite. A cross-section of the conductor is shown with the current direction indicated by a dot. You should show give an indication of field shape, direction & relative field strength.Magnetic Fields
Electric Power RevisionQuestion Type:Two wires carry current in opposite directions as shown in the diagram below. The current in wire Y is twice the current in wire X. Point Q is midway between wires X and Y.Use the following key for your answers:A. To the rightB. To the leftC. UpD. DownE. Into the pageF. Out of the pageG. Zero
Q3: Which of the following best describes the direction of the resultant magnetic field at point QQ4: The current in wire X is reversed. Both conductors now have current passing from right to left. Which alternative would now represent the resultant magnetic field?Magnetic FieldsInto page - E Into page - E
1.3 Magnetic Fields around SolenoidsThe strength of this central magnetic field can be increased by filling the space in the centre of the cylinder with magnetically susceptible material, eg. Soft ironA SOLENOID is, by definition, a series of loops of wire placed side by side to form a coil.In reality, solenoids are produced by winding a single piece of wire around a cylindrical former.When a current flows through the wire, a strong, uniform magnetic field is produced down the centre of the cylinder.The solenoid remains a magnet while the current continues to flow. This is a so called ELECTROMAGNET. Solenoid split down its centre lineStrong FieldStronger Field
1.4 The North Pole of a SolenoidThumb points to the NORTH pole of the solenoidFingers CURL in the direction ofthe CURRENT though the coil.In this form the rule is: Electromagnets generate magnetic fields. Magnetic fields have of a North and South pole.So an electromagnet must have a North and a South pole.How do you determine which end of the electromagnet is North ?Its easy using a modified version of the Right Hand Grip Rule
Electric Power RevisionQuestion Type:The following diagram shows a simple generator, which consists of coil R that can be rotated in a magnetic field. Electric contact is made with the coil through a pair of slip rings. The magnetic field is produced by passing a DC current through two fixed coils wound on two iron pole pieces to form an electromagnet. The magnetic field strength is 0.1 T. The rotating coil R has an area of 4.0103 m2 . It consists of 40 turns of wire.Q5: What is the direction of the magnetic field passing through coil R?A To the leftB To the rightC UpD Down
Magnetic field direction
Electric Power RevisionQuestion Type:Figure 1 below shows a solenoid powered by a battery.Q6: Complete the diagram above by sketching five magnetic field lines created by the solenoid.Make sure that you clearly show the direction of the field, including both inside and outside the solenoid.SolenoidsA common error was not indicating the direction of the field both inside and outside the coils. Other errors included the field lines not being continuous or crossing one another, or a number of lines joining into one.
Electric Power RevisionQuestion Type:A magnet is moved through a coil at constant speed and out the other side.Q7: Which one of the diagrams (A D) best shows how the current through the coil varies with time?Induced Current
Electric Power RevisionQuestion Type:The solenoid in the Figure is merely a series of coils lined up parallel to each other so that each of the individual coil's magnetic fields add together to produce a stronger magnetic field.Q8: Describe two ways that we could further increase the magnetic field strength within the solenoid?any 2 of: Insert a ferromagnetic material (eg soft iron bar) Decrease the diameter of the solenoid Increase the current into (or decrease the resistance of) the solenoid
Magnetic Field Strength
Chapter 2Topics covered:Magnetic Field StrengthMagnetic InteractionsMagnetic Force on a Current Carrying WireThe Right Hand Palm RulePalm Rule ApplicationsMagnetic Force on a Moving Charge
2.0 Magnetic Field StrengthMagnetic Field RepresentationsField Out of PageField Into PageStrongWeakStrongWeakTo fully describe the strength of a Magnetic Field at any point, both a magnitude and direction need to be specified. Thus the Magnetic Field Strength is a VECTOR quantity.
This vector is actually called the MAGNETIC FLUX DENSITY, symbol B, unit TESLA (T). However the vector is often (incorrectly) labelled the Magnetic Field Strength.
Will generate a strong Magnetic ForceWill generate a weak Magnetic Force
2.1 Magnetic InteractionsField due to Permanent MagnetA current carrying wire is placed in an external magnetic field. The magnetic field surrounding the wire and the external field interact to produce a FORCE which is experienced by the wire.The force will cause the wire to move away from the area of high field toward the area of low fieldSo the wire will experience a force up the page
2.2 Magnetic Force on a Current carrying WireThe direction of the Force is determined by using the Right Hand Palm Rule. (see next slide)where: FMAG = Magnetic Force (N) I = Current (A) L = Length of Wire (m) B = Magnetic Flux Density (T)The SIZE of the Force experienced by the wire is determined from: FMAG = ILB
2.3 The Right Hand Palm RuleN.B. The FORCE is MAXIMUM WHEN THE EXTERNAL FIELD (B) is PERPENDICULAR to the CURRENT (I). N.B. The FORCE is ZERO when the FIELD and CURRENT are PARALLELThe FINGERS point in the direction of the EXTERNAL FIELD.The THUMB points in the direction